Quantitative light microscope autoradiography of (3H) ouabain, a specific inhibitor of Na-K ATPase, was employed to measure the distribution of Na-K transport sites in the photoreceptor layer of Rana pipiens. Excised retinas were exposed from 0-30 min to variable concentrations of (3H) ouabain, washed in drug free medium and processed for autoradiography using freeze-dry, vacuum-embedding techniques. Grain density analyses showed that saturation binding to photoreceptors was achieved after 30 and 5 min exposures to 2 and 20 microM ouabain respectively. From measurements of saturation binding in 4 animals and at concentrations ranging from 2 - 40 microM, it was calculated that each rod possesses about 12.5 X 10 to the 6th power Na-K transport sites. Nearly all of these are located on the inner segments. Autoradiography of isolated outer segments confirmed this distribution. The results of this study show that all of the rod dark current can be accounted for by inner segment Na-K pumps. A high level of binding to the inner plexiform and optic nerve layers was observed also but not analyzed quantitatively.